Priority is claimed to Patent Application Number 2001-8011 filed in the Republic of Korea on Feb. 17, 2001, herein incorporated by reference.
1. Field of the Invention
The present invention relates to initialization of a digital subscriber line, and more particularly, to an initializing method for Multi-Carrier Modulation (MCM) Very high bit rate Digital Subscriber Line (VDSL), and a system supporting the method.
2. Description of the Related Art
FIG. 1 is a diagram for explaining an initialization process when an ordinary 4.3125 kHz tone space is used. A sequence on the left specifies a Special Operation Channel (SOC) which is exchanged between a VDSL Transceiver Unit at the Optical network unit (VTU-O) and a VDSL Transceiver Unit at the Remote site (VTU-R). A sequence on the right specifies a symbol type which is exchanged between VTU-O and VTU-R.
An initialization process in a VDSL, especially in a Multi-Carrier Modulation (MCM) VDSL, can be divided into a handshake process and an actual initialization process. As shown in FIG. 1, immediately after a G.994.1 handshake process is finished, an actual initialization process having a 4.3125 kHz tone space starts. In FIG. 1, just the front part of the actual initialization process is shown.
In the present VDSL standard, a space between tones (briefly, a tone space) is described as 4.3125 kHz. A 8.625 kHz tone space, twice the tone space of the present VDSL, may be used as an option. When the 8.625 kHz tone space is used, it has an advantage of using two times wider frequency band with the same Fast Fourier Transform (FFT) size. The frequency band which is currently used in the VDSL standard is 138 kHz˜12 MHz. The band equal to or less than 138 kHz may be used as an option, and the band equal to or greater than 12 MHz is described as the object of future study. When a default FFT size which is 4096 points (always twice the number of tones being used) is used, the frequency band which can be used for VDSL data communications is limited to 8.832 MHz (4.3125 kHz*2048). When a 8192 point FFT size is used, it is clear that a 17.664 MHz band can be used. However, due to difficulties in realizing an FFT core (for example, complexity, speed, etc.), doubling a tone space is described as an option in the VDSL standard. When an 8.625 kHz tone space is used, a frequency band that is twice as wide can be used with the same FFT size, and therefore enhances data rate when the length of a loop is short, that is, when data can be transmitted even in a high frequency band.
In the 8.625 kHz tone space option described in the VDSL standard, in order to use the 8.625 kHz tone space, prior art handshaking and actual initialization process is performed once. Then, if it is determined according to the measured length of the loop that the 8.625 kHz tone space can improve the data speed, the same handshaking and actual initialization process is performed again based on the 8.625 kHz tone space. In this case, since the same initialization process is performed twice, the time for initializing a modem is doubled.